include/linux/fs.h: disable preempt when acquire i_size_seqcount write lock

Two rt tasks bind to one CPU core.

The higher priority rt task A preempts a lower priority rt task B which
has already taken the write seq lock, and then the higher priority rt
task A try to acquire read seq lock, it's doomed to lockup.

rt task A with lower priority: call write
i_size_write                                        rt task B with higher priority: call sync, and preempt task A
  write_seqcount_begin(&inode->i_size_seqcount);    i_size_read
  inode->i_size = i_size;                             read_seqcount_begin <-- lockup here...

So disable preempt when acquiring every i_size_seqcount *write* lock will
cure the problem.

Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 2c28271ab9d4..17d8b1596215 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -675,9 +675,11 @@ static inline loff_t i_size_read(const struct inode *inode)
 static inline void i_size_write(struct inode *inode, loff_t i_size)
 {
 #if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+        preempt_disable();
         write_seqcount_begin(&inode->i_size_seqcount);
         inode->i_size = i_size;
         write_seqcount_end(&inode->i_size_seqcount);
+        preempt_enable();
 #elif BITS_PER_LONG==32 && defined(CONFIG_PREEMPT)
         preempt_disable();
         inode->i_size = i_size;